During the repair of coated metal articles, primarily after they have been used in operation but sometimes after damage during manufacture, generally it is necessary first to remove the coating to expose the substrate for application of a repair technique. For example, coated gas turbine engine components, particularly high temperature operating parts such as turbine blades, vanes, nozzles, and combustors, frequently are protected from erosive, corrosive, and oxidation environments by coatings. Generally such coatings are metallic in nature, although ceramic or cermet-type coatings have been used.
During operation of such gas turbine engine components, environmental wear and attack as well as damage from foreign airborne objects can occur, particularly to those component portions upon which air or combustion product passing through the engine impinges. Prior to repair of such components, generally it is desirable to remove the coating with little or no damage to the substrate material. One commonly used method for coating removal, sometimes referred to as "grit blasting", involves impacting the coating with abrasive particles, frequently aluminum oxide. Commonly, such coating removal is conducted in an enclosure lighted by incandescent or fluorescent lights. Generally, the enclosure includes a viewing window and protected access ports for use by an operator in conducting the coating removal. In one form, the operator holds the article to be treated in rubber gloves disposed in the access ports while a grit blast nozzle projects abrasive against the coated surface to remove coating.
Under such conditions, it has been found that it is difficult to determine the point at which all coating material has been removed from a substrate which visually appears to be substantially the same as the coating. Excessive impacting of the article substrate by the abrasive particles after coating removal has resulted in damage or excessive substrate removal. Such a problem in distinguishing a coating from its substrate is particularly difficult in the case of metallic coatings on metallic substrates.